As a large amount of fossil fuels, such as petroleum, have been used hitherto, the accompanying increase in the concentration of CO2 has given rise to global warming as a serious problem, and further, there has been a concern of depletion of the fossil fuels. Therefore, it has become a very important global issue as to how to meet the large amount of energy demand in the future. Under these circumstances, it has been studied actively to use clean solar light, which is infinite and does not generate harmful materials like nuclear power generation, for power generation. As a solar cell that converts light energy into electrical energy, inorganic solar cells such as a crystal silicon-based solar cell, a polycrystalline silicon-based solar cell, and an amorphous silicon-solar cell; and organic solar cells using an organic dye or a conductive polymer material, have been proposed.
Under these circumstances, a dye-sensitized solar cell (Graetzel type solar cell) (Non-Patent Document 1 and Patent Document 1) suggested by Graetzel et al., Switzerland, 1991, is anticipated as a next-generation solar cell since it is capable of providing a conversion efficiency at the same level as amorphous silicon using a simple and easy preparation process. The Graetzel type dye-sensitized solar cell includes a semiconductor electrode in which a semiconductor layer having a dye adsorbed thereon is formed on a conductive substrate; a counter electrode opposed to the electrode and formed of a conductive substrate; and an electrolyte layer held between both electrodes.
In this cell, the adsorbed dye absorbs light and then enters an excited state, and the electrons are injected into the semiconductor layer from the excited dye. The dye that had been in an oxidized state due to the discharge of the electrons returns to the original dye when the electrons move to the dye by an oxidation reaction of a redox agent in the electrolyte layer. Further, the redox agent which donates electrons to the dye is reduced again in the counter electrode. By such a series of the reactions, the function of a cell is realized.
In the Graetzel type dye-sensitized solar cell, the effective reaction surface area may be increased by about 1000 times by using porous titanium oxide formed by sintering fine particles in the semiconductor layer, thereby extracting a larger photocurrent than solar cells of the related art. In the Graetzel type dye-sensitized solar cell, a ruthenium complex is used as a sensitive dye, and specifically, a bipyridine complex of ruthenium, such as a cis-bis(isothiocyanato)-bis-(2,2′-bipyridine-4,4′-dicarboxylic acid) ruthenium (II) bi-tetrabutylammonium complex, cis-bis(isothiocyanato)-bis-(2,2′-bipyridine-4,4′-dicarboxylic acid) ruthenium (II), or a terpyridine complex of ruthenium, such as tris(isothiocyanato) (2,2′:6′,2″-terpyridyl 4,4′,4″-tricarboxylic acid) ruthenium (II) tri-tetrabutylammonium complex, is used.
Further, in Patent Document 6, a novel merocyanine dye and a preparation method therefor are described. Further, in Patent Document 7, a semiconductor for a photoelectric conversion material, which includes a predetermined heterocyclic compound, is described.